Download NON DOUBLE COUPLE EARTHQUAKES IN EURASIA

NON DOUBLE COUPLE EARTHQUAKES IN EURASIA - NORTH
AMERICAN INTERPLATE BOUNDARY
S.V. Baranov, A.N. Vinogradov
Kola Branch of Geophysical Survey of RAS, Apatity, Russia
[email protected]
Baranov S.V., Vinogradov A.N.
Non Double Couple Earthquakes in Eurasia - North American Interplate Boundary
The research database contains 164 earthquakes with 4.7 ≤ Mw ≤ 6.7 taken from CMT
catalog and occurred in the rectangle of 72° - 90° N and 7°W - 120°E in 1977-2011. A scope of
earthquakes occurred in Mon, Knipovich and Gakkel mid-ocean ridge belts (MORB) has been
made, and distribution of double-couple (DC) and non-double-couple (NDC) earthquake sources
has been evaluated using Lode-Nadai coefficient: LN=3M2/(M1−M3), where M1, M2, M3 are
principal values of seismic moment tensor Mij (M1≥M2≥M3), (abs(LN)≤1 ). If LN = 0, the strain
in a source is a pure shear. If LN = 1 (-1), the strain is a uniaxial compression (tension). We
suppose that compression (tension) strains are controlled by oceanic bottom spreading which
causes a volcanic eruption. These processes are traditionally supposed to control seismicity in
MORBs.
Moment tensor components in CMT catalog have errors (δMij) which essentially change
LN-value. To take them into account we simulated moment tensors for 106 uncertainties applied
to 5 independent tensor components uniformly distributed within (Mij - δMij, Mij + δMij). The 6th
independent component was calculated by means of zero trace condition. If determinants of the
disturbed moment tensors have different signs we refer an event to DC one (LN = 0), otherwise
an event has NDC part (LN ≠ 0). Contribution of NDC part to a scalar moment M0 of an event is
proportional to LN-value: M0(NDC) = M0·abs(LN).
As revealed, the DC events prevail among earthquakes in all MORBs. In Gakkel Ridge
with ultra low speed of spreading the NDC earthquakes, caused by normal rifting or volcanic
eruptions, are responsible for 4.9% of seismic moment release only. Whereas in the Mon and
Knipovic Ridges NDC contribution is 4 times higher. Thus, the seismological data give
evidences that the Arctic segment of the interplate boundary between Eurasian and North
American lithosphere plates belongs recently to the transform geodynamic type, and only in the
North Atlantics (within the Mon Ridge) a divergent type is still alive.
Tectonic situation and earthquake genesis in the Knipovich and Gakkel Ridges, which
form the NW and Northern segment of border between Eurasian and North American
lithospheric plates, require more detailed investigation. The pattern of NDC earthquakes spatial
distribution in these areas does not correspond to the geodynamic models which explain that a
regional seismicity is controlled by spreading of ocean bottom in the North Atlantic and in
Arctic, because the major contribution to the total seismic moment release in the Arctic MORBs
caused by shear dislocations.